Fixing Slepton MG Control and adding gridpack

Description of bug

Simulation jobs were crashing because of the following error: ISF_InputConverterWARNING Unable to find mass of particle with PDG ID '2000015' in ParticleDataTable. Will set mass to generated_mass: 50000 ISF_InputConverter ERROR ISF_to_G4Event particle conversion failed. ISF_Particle PDG code = 2000015

see request https://its.cern.ch/jira/browse/ATLMCPROD-9056

Changes introduced

Configuring the Stau sleptons so that the particules could be used in simulation and adding gridlocks so that the jobs could run faster.

Issues resolved

Needed for ATLMCPROD-9056